I imported the upgrade to iPhoto. Now, the upgrade is saying this:
The photo library needs to be upgraded to work with this version of iPhoto.
Your photo library will not be readable by previous versions of iPhoto after the upgrade. The upgrade process for very large libraries may take an hour or more to complete.
I do not want to lose my older pictures; someone please advise me.
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Good question! The short answer is that no, you will not lose your older pictures. The upgrade just reformats the library to work only with the latest version of iPhoto, so other versions of the software will not be able to open it. After the upgrade, if you want those photos available for another program (or a previous version of iPhoto, you'll need to export them.
I hope that helps. If you have any other questions, I recommend iPhoto: The Missing Manual
Senior Editor, Missing Manuals